1. Field of the Invention
This invention relates in general to the coating art and in particular to the coating of supports with liquid coating compositions. More specifically, this invention relates to an improved method of coating sheet materials, such as webs composed of synthetic organic polymers or of polymer-coated paper, with coating compositions comprising a film-forming material in a liquid vehicle, whereby the formation of mottle in the coated layer is reduced.
2. Description of the Prior Art
Formation of mottle in the coating of supports with liquid coating compositions is a very common problem which is encountered under a variety of circumstances in the coating arts. For example, coating compositions consisting of solutions of a polymeric resin in an organic solvent are frequently coated in layer form onto sheet materials, such as webs of synthetic organic plastic material. Mottle, or non-uniform density, is an especially severe problem when the coating solvent is a volatile organic solvent but can occur to a significant extent even with aqueous coating compositions or with coating compositions utilizing an organic solvent of low volatility. The mottle is an undesirable defect in some instances because it detracts from the appearance of the finished product and in some instances, such as in the photographic art, it is also undesirable because it adversely affects the functioning of the coated article. Various expedients have been employed heretofore in an effort to eliminate, or at least minimize, the formation of mottle in coated layers. For example, surfactants are often added to the coating compositions as described, for example, in U.S. Pat. No. 3,514,293. These are sometimes effective in reducing mottle but in many cases the degree to which mottle forms is still excessive in spite of the inclusion of a surfactant in the coating composition. It is believed that there are a variety of factors which can contribute to the formation of mottle and the exact mechanism of its formation is not well understood. Regardless of the specific causes of mottle, its formation in coated layers, as well as the occurrence of other defects such as streaks and lines, is a long standing problem of serious concern in the manufacture of coated materials, and especially in the manufacture of photographic products.